In this study, transient temporal-spatial evolutions of femtosecond (fs) laser pulse-induced filaments and electronic plasma when laser induced damage occurred in fused silica were investigated using fs time-resolved pump-probe shadowgraphy. The transient peak electron density increased and then decreased as delay time of probe beam increased. Its corresponding spatial positions moved from the sample surface to the inside of the sample, but remained at the nonlinear focus for a relatively long time. The maximum electron density increased as pump energies increased and then became saturated at 8 μJ, above which laser-induced material damage occurred. The material damage threshold electron density was approximately 1.27×10²⁰ cm^-3. The laser-induced material damage position corresponded to the position of the maximum electron density. Furthermore, the material damage was extended from the nonlinear focus to the deeper parts of the sample at pump energies above 8 μJ. This tendency agreed well with the spatial distribution of the maximum transient electron density at each propagation depth, implying that the fs time-resolved pump-probe shawdowgraphy is a meaningful tool for predicting the distribution of laser-induced microstructures in ultrafast laser micromachining.

中文翻译：

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飞秒激光诱导的损伤中电子等离子体的时空动态

在这项研究中，使用fs时间分辨的泵浦探针阴影成像技术研究了飞秒（fs）激光脉冲诱导的细丝和电子等离子体在熔融石英中发生激光损伤时的瞬时时空演变。瞬态峰值电子密度随探测束延迟时间的增加而增加，然后降低。其相应的空间位置从样品表面移至样品内部，但在非线性焦点上保持了相当长的时间。最大电子密度随泵浦能量的增加而增加，然后在8μJ时达到饱和，在此之上发生激光诱导的材料破坏。材料破坏的阈值电子密度约为1.27×10 ²⁰ cm ^-3。激光引起的材料损坏位置对应于最大电子密度的位置。此外，在泵浦能量大于8μJ时，材料损伤从非线性焦点扩展到了样品的较深部分。这种趋势与每个传播深度处最大瞬态电子密度的空间分布非常吻合，这意味着fs时间分辨的泵浦探针影像学是预测超快激光微加工中激光诱导的微结构分布的有意义的工具。